Proximal sesamoid bone (PSB) fractures are the highest reported cause for catastrophic injuries in Thoroughbred racetracks in North America and Hong Kong. The fracture etiology is poorly understood and has been researched extensively, yet it is unclear whether fractures occur as acute, traumatic monotonic fractures, versus are the result of chronic, cyclical bone stress with inadequate secondary bone remodeling. The goal of this research is to determine whether there are differences in the amount of microdamage accumulation in PSBs in horses sustaining fracture as compared to sex-age-matched controls (SAMC), to assess whether intact PSBs from the contralateral limb of fracture horses (FXCL) have reduced fracture toughness, and to investigate the amount of microfracture that occurs during fracture testing. Our first hypothesis is that PSBs undergoing catastrophic failure will have evidence of pre-existing microdamage. To test this hypothesis, microdamage will be assessed in FXCL PSBs and PSBs from SAMCs using lead uranyl acetate (UA) staining and micro-CT; with microarchitecture evaluated via basic fuchsin staining. Second, fracture toughness is thought to be higher in more porous bone as it facilitates dissipation of fracture energy in the form of microfracture. Henceforth, we postulate PSBs from FXCL will have an increase in microfracture undergoing three-point-bending fracture toughness testing when compared to SAMC. Following dissection, imaging, and morphometric measurements of PSBs, fracture toughness will be quantified using in vitro mechanical testing. Fracture toughness will be compared between FXCL and SAMC, and will be correlated with morphometric measurements, bone density, and microfracture volume as measured via CT and micro-CT.